As the use of electronic devices, such as personal computers, continues to increase, it is becoming ever more important to make such devices portable. The usefulness of portable electronic devices, such as notebook computers, is limited by the limited length of time batteries are capable of powering the device before needing to be recharged. This problem has been addressed by attempts to increase battery life and attempts to reduce the rate at which such electronic devices consume power.
Various techniques have been used to reduce power consumption in electronic devices, the nature of which often depends upon the type of power consuming electronic circuits that are in the device. For example, electronic devices such as notebook computers, typically include memory devices, such as dynamic random access memory (“DRAM”) devices, that consume a substantial amount of power. As the data storage capacity and operating speeds of memory devices continue to increase, the power consumed by such devices has continued to increase in a corresponding manner. Therefore, many attempts to reduce the power consumed by an electronic device have focused on reducing the power consumption of memory devices.
In general, the power consumed by a memory device increases with both the capacity and the operating speed of the memory device. The power consumed by memory devices is also affected by their operating mode. For example, a DRAM device generally consumes a relatively large amount of power when the memory cells of the DRAM device are being refreshed. As is well-known in the art, DRAM memory cells, each of which essentially consists of a capacitor, must be periodically refreshed to retain data stored in the DRAM device. Refresh is typically performed by essentially reading data bits from the memory cells in each row of a memory cell array and then writing those same data bits back to the same cells in the row. A relatively large amount of power is consumed when refreshing a DRAM because rows of memory cells in a memory cell array are being actuated in the rapid sequence. Each time a row of memory cells is actuated, a pair of digit lines for each memory cell are switched to complementary voltages and then equilibrated. As a result, DRAM refreshes tend to be particularly power-hungry operations. Further, since refreshing memory cells must be accomplished even when the DRAM is not being used and is thus inactive, the amount of power consumed by refresh is a critical determinant of the amount of power consumed by the DRAM over an extended period. Thus many attempts to reduce power consumption in DRAM devices have focused on reducing the rate at which power is consumed during refresh.
Refresh power can, of course, be reduced by reducing the rate at which the memory cells in a DRAM are being refreshed. However, reducing the refresh rate increases the risk that data stored in the DRAM memory cells will be lost. More specifically, since, as mentioned above, DRAM memory cells are essentially capacitors, charge inherently leaks from the memory cell capacitors, which can change the value of a data bit stored in the memory cell over time. However, current leaks from capacitors at varying rates. Some capacitors are essentially short-circuited and are thus incapable of storing charge indicative of a data bit. These defective memory cells can be detected during production testing, and can then be repaired by substituting non-defective memory cells using conventional redundancy circuitry. On the other hand, current leaks from most DRAM memory cells at much slower rates that span a wide range. A DRAM refresh rate is chosen to ensure that all but a few memory cells can store data bits without data loss. This refresh rate is typically once every 64 ms. The memory cells that cannot reliably retain data bits at this refresh rate are detected during production testing and replaced by redundant memory cells.
One technique that has been used to prevent data errors during refresh as well as at other times is to generate an error correcting code “ECC,” which is known as a “syndrome,” from each item of stored data, and then store the syndrome along with the data. When the data are read from the memory device, the syndrome is also read, and it is then used to determine if any bits of the data are in error. As long as not too many data bits are in error, the syndrome may also be used to correct the read data.
The use of ECC techniques can allow DRAM devices to be refreshed at a slower refresh rate since resulting data bit errors can be corrected as long as the refresh rate is not so low that more errors are generated than can be corrected by ECC techniques. The use of a slower refresh rate can provide the significant advantage of reducing the power consumed by DRAM devices. Prior to entering a reduced power refresh mode, each item of data is read. A syndrome corresponding to the read data is then generated and stored in the DRAM device. When exiting the reduced power refresh mode, the each item of data and each corresponding syndrome are read from the DRAM device. The read syndrome is then used to determine if the item of read data is in error. If the item of read data is found to be in error, the read syndrome is used to correct the read item of data, and the incorrect item of data is then overwritten with the corrected item of data.
One disadvantage of using the above-described ECC techniques in memory systems is the time and power required to generate and store ECC syndromes when entering the reduced power refresh mode. Each time the reduced power refresh mode is entered, all of the data stored in the DRAM device must be read, and a syndrome must be generated for each item or group of items of read data. The generated syndromes must then be stored. It can require a substantial period of time to accomplish these operations for the large amount of data stored in conventional high-capacity DRAM devices. During this time that the stored data are being checked, the DRAM device generally cannot be accessed for a read or a write operation. As a result, the operation of memory access devices, such as processors, is stalled until the data checking operations have been completed. Furthermore, a substantial amount of power can be consumed during the time the stored data are being checked and possibly corrected. These operations must be performed even though very little if any of the data stored in the DRAM device may have changed since the data was previously read and corresponding syndromes stored.
A similar problem exists where ECC techniques are being used to correct data storage errors in normal operation, i.e., not for a reduced power refresh mode. Each time a read request is coupled to a DRAM or other memory device, the syndrome corresponding to the read data must also be read, and the read data must then be checked using the read syndrome. These operations must be performed each time a read request is received even though the read data may not have changed since the read data was either written or previously read. The time required to perform these operations increases the latency of the memory device since the read data are not accessible to a memory requester until after these operations have been completed.
There is therefore a need for a memory system and method that uses ECC techniques to insure data integrity and allow operations in a reduced power refresh mode, but does so in a manner that does not unduly increase the read latency or power consumption of the memory device.